Crust‐mantle decoupling by flexure of continental lithosphere

Abstract

Mechanical decoupling between the crust and upper mantle has been proposed as an explanation for anomalously low effective elastic thicknesses (<20 km) locally associated with thermally mature (∼1 Ga) continental lithosphere. The processes and consequences of crust‐mantle decoupling are investigated with a fully dynamic elastic‐viscous‐plastic (EVP) finite element model of orogenic loading and foreland basin subsidence. The basic dependencies of continental flexure on lithospheric thermal state, crustal thickness, and load magnitude are determined and are characterized by the best fit elastic thickness for the simulated deflections. Additional variables such as loading rate, viscoelastic stress relaxation, the boundary condition at the underthrust edge of the plate, and the lithospheric rheology also influence the flexural signature, and these sensitivities are likewise defined by EVP simulations. Decoupling can result in substantial reductions in the effective elastic thickness‐up to a factor of 2 for lithosphere with a thermal age of 1 Ga. Elastic‐viscous‐plastic models show that lower crustal weakening occurs by locally enhanced creep driven by high shear stresses in the middle lithosphere at the load margin. Crust‐mantle decoupling in these models is fundamentally controlled by the temperature and the rheological contrast at the Moho. Previous studies of flexural decoupling have employed simplified multilayered elastic or elastic‐plastic methods, the latter using a yield strength envelope with aprescribed strain rate and an assumption of complete slip at the Moho. However, lithospheric rheology is inherently stress and time dependent, with the deformation rate varying both spatially and temporally. Comparison of elastic‐plastic solutions to the EVP simulations indicates that the former method is satisfactory for oceanic and coupled continental lithosphere but performs poorly with decoupled lithosphere. The EVP model is marginally successful at achieving effective elastic thicknesses <20 km for ∼l‐Ga lithosphere; additional reductions in the thickness may be achieved by a more thorough treatment of the thermal evolution of underthrust continental lithosphere.